Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: daveyajd on February 19, 2012, 06:09:35 pm
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I have a 3 prong installed on my Fender but it is old and fraying and the "death cap" is still installed so I am going to wire a new one in. I came across this info on guitarfiles.com. Thank you to the creator.
(http://i422.photobucket.com/albums/pp301/drivenbyastronauts/Amp%20Pictures/Fender_3_Prong_Layout.jpg)
I want to eliminate the courtesy plug and ground switch from the circuit. Maybe I'll use the switch location for NFB switch later on and the plug location as a place to measure my cathode current for biasing. Will these changes in wiring work?
(http://i422.photobucket.com/albums/pp301/drivenbyastronauts/Amp%20Pictures/Fender_3_Prong_Annotated.jpg)
I'm thinking "hot" wire to the fuse center. And then just connecting and heat shrinking the connection between PT and "neutral" wire. Then grounding green of course. I know the power cord upgrade is common practice but I never hear of anybody eliminating these parts. Anybody done it?
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I'm thinking "hot" wire to the fuse center. And then just connecting and heat shrinking the connection between PT and "neutral" wire. Then grounding green of course.
That's perfect.
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And exactly how I've done it in the past.
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Thanks for all the answers. It only took a few minutes to do. Here are the before and after shots:
(http://i422.photobucket.com/albums/pp301/drivenbyastronauts/Amp%20Pictures/SAM_2830.jpg)
(http://i422.photobucket.com/albums/pp301/drivenbyastronauts/Amp%20Pictures/SAM_2831.jpg)
How would I wire up a MV using that switch location?
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It might work, but I'd be hesitant to mount a master volume next to power a.c. wiring like that, because of the likelihood of hum coupling into your volume control.
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Ok. Could I use a pot for the NFB in there? The schematic calls for an 820 resistor in there but I have seen higher values so would I use a 1 K linear? 5k?
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Alright, since I was in there and the iron was hot I went ahead and did some other changes I have been thinking about. Changed the V2 plate resistors, orange drop PS in place of the Mallorys in the tone stack, silver micas replaced the treble cap and bright switch ceramics. Then I moved to the PI. Changed the 47k plate resistors to blackface spec 100k,82k. Changed the coupling cap to .001 instead of 680pf. The only thing I haven't done yet is change these resistors to 220k instead of 100k.
(http://i422.photobucket.com/albums/pp301/drivenbyastronauts/Amp%20Pictures/PIannotated.jpg)
The plate resistors for the power tubes right? What will a change to blackface value yield here? I believe the 47k I replaced for the PI were reducing the gain in that stage. Should I expect the same here?
Also, my plate voltage seems to be low compared to schematics and similar model amps people have posted about. I'm about 402v at 42-46mA. Maybe I will put a GZ34 in there to bump it up. The amp is definitely sounding better with each change.
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It might work, but I'd be hesitant to mount a master volume next to power a.c. wiring like that, because of the likelihood of hum coupling into your volume control.
i even mounted an input next to the AC wiring and had no hum issues. but i added a metal plate between the two as a shield.
before i gutted my Super Twin Reverb a shield like that was already there to separate the transformer wiring from the rest of the circuit. i kept it as you can see in this picture. (and cut out a small part for the heater wiring)
(http://img85.imageshack.us/img85/2433/dsc04899p.jpg)
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The only thing I haven't done yet is change these resistors to 220k instead of 100k.
The plate resistors for the power tubes right? What will a change to blackface value yield here?
Those are the grid bias resistors for the output tubes. Changing to 220K will have less loading effect on the PI plates. Gain will increase slightly, but may not be perceivable.
Which amp is this?
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Ok. Could I use a pot for the NFB in there? The schematic calls for an 820 resistor in there but I have seen higher values so would I use a 1 K linear? 5k?
Yes, you could use a NFB pot there. My reasoning is the low impedance of that circuit makes it less susceptible to hum.
Since 820 is the stock resistor value, you'd probably want at least a 5k pot there, maybe larger. Also, you might want a smallish resistor in series with the pot (in place of the 820 ohm resistor); that would set an upper limit for the amount of NFB. If the series resistor equals 820 ohms, the stock amount of feedback is the most you could get.
i even mounted an input next to the AC wiring and had no hum issues. but i added a metal plate between the two as a shield.
Slick idea, Birt!
And that'll generally work; I assumed his question meant "is it okay?" without extra measures such as shielding.
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Birt is that a compactron tube socket in that amp?
Brad :icon_biggrin:
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Those are the grid bias resistors for the output tubes. Changing to 220K will have less loading effect on the PI plates. Gain will increase slightly, but may not be perceivable.
Which amp is this?
Grid bias resistors. Yeah that came to me in my sleep last night. Thanks.
This is a Bantam Bass amp. I think I may just change those out next time I have a second.
With plate voltages this low JJ 6v6s might work. I know they are rated for higher than normal plate voltages. Maybe I could get some lower volume overdriven sounds. Anyone used those in old 6L6 amps?
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Birt is that a compactron tube socket in that amp?
yes. the super twin reverb had a 6C10.